Autologous mouse testicular cells derived from the seminiferous tubules activate suppressor T cells which inhibit mixed cell reactions in vitro. Generation of cytotoxic T cells in vitro is reduced in the presence of syngeneic germ cells (spermatozoa from the siminiferous tubules or epididymal sperm). Spleen cells from mice injected with syngeneic sperm show a nonspecifically suppressed potential to generate cytotoxic T cells in vitro. These observations may be relevant for the recently observed immunosuppressed condition seen among male homosexuals, and raises the possibility that the sperm is a contributing factor to this suppression.